1. Field of the Invention
This invention relates to a distance measuring device and more particularly to the computing processing circuit of a distance measuring device for a camera.
2. Description of the Prior Art
Cameras have been known which use a distance computing method in which a distance from the camera to the object to be photographed is computed from the ratio of two different distance information signals, varying with the object distance. A camera of this kind is provided with a circuit for obtaining the actual ratio of the above different distance information signals. For such a circuit, there is known a digital method in which each of these different distance information signals is analog-to-digital (A-D) converted and the ratio is computed by means of shift registers, etc. In another known method for the same purpose, a subtracting operation is carried out with the two different distance information signals respectively logarithmically compressed. However, the first method, which necessitates use of A-D converters, results not only in a large circuit scale but also in a long time period required for A-D conversion and, thus, takes a long time period for distance measurement because of a slow computing process. Furthermore, when using a distance measuring device of the so-called active type, a light signal corresponding to the brightness of the object to be photographed varies to a great extent with the distance from the distance measuring device to the object and thus requires a large dynamic range for the signal processing circuit of the device and a large number of bits for the A-D converter. Therefore, in that situation, the circuit scale increases further and the A-D conversion time becomes longer. The above second method in which distance information values are converted into logarithmic values and their ratio is obtained by subtraction results in a large error, which makes variations in the small ratio value hardly discernible. In the distance measuring device for a camera in particular, the power source is a dry cell and the dynamic range of the element outputs in a signal processing circuit is only 1.5 to 3.0 V or thereabouts. Furthermore, the offset voltage of amplifiers included in the signal processing circuit further narrows the dynamic range. To solve this problem, some known distance measuring devices are provided with booster circuits for widening the dynamic range. However, this arrangement not only results in a large circuit scale but also increases the cost of the device.
It is therefore an object of this invention to provide a distance measuring device which eliminates the shortcomings of the prior art devices.
This and other objects and features of the invention will become apparent from the following detailed description of the preferred embodiments thereof taken in conjunction with the accompanying drawings.